1. Field of the Invention
This invention relates to ferroelectric liquid crystal displays and compounds and mixtures for use in such displays.
More particularly, the invention relates to a liquid crystal display cell having a chiral ferroelectric smectic liquid crystal layer whose helix structure is so influenced by the action of an electric field that its optical anisotropy alters; a pair of transparent plates between which the liquid crystal is disposed and which have a surface structure aligning the molecules of the liquid crystal and are provided with electrodes to produce an electric field therein; and one polariser each before and after the liquid crystal. The invention also relates to chiral or optically active compounds and liquid crystal mixtures for use in the cell and the use of the compounds for electro-optical purposes.
2. Description
The effect of distorting a helix structure of a ferroelectric liquid crystal cell hereinbefore set out by an electric field has been described in Ostrovski, B. I., Advances in Liquid Crystal Research and Applications, Oxford/Budapest, 1980, pp. 469 et seq. The effect is sometimes referred to in the literature by the abbreviation DHF for xe2x80x9cdistorted helix ferroelectricxe2x80x9d. The article of Ostrovski considers the possibility of modulating a light beam at a frequency of 1 kHz and using an electric field of less than 6xc3x97103 V/cm. Subsequently, however, it was apparently found that this effect does not lead to an electro-optical cell which is usable in practice.
A known effect, but of a different kind, is Lagermall and Clark""s surface stabilized ferroelectric liquid crystal (SSFLC) in which ferroelectric liquid crystals are used in an untwisted configuration. This effect has been described in Mol. Crystal. Liq. Crystal. 94 (9183), 213-234 and 114 (1984), 151-187.
As described hereinbelow, the present invention provides a distorted helix ferroelectric (DHF) cell which has not only the known advantages of the DHF effect but also the properties necessary for practical use as a light shutter or display cell.
According to the invention, this is achieved by a DHF cell of the kind hereinbefore set out wherein the ratio d/p of the thickness d of the liquid crystal layer to the pitch height p of the helical twisting is greater than 5, the smectic tilt angle xcex8o is between 22.5xc2x0 and 50xc2x0 and the product dxc2x7xcex8o 2xc2x7xcex94nxc2x71/xcex is greater than 0.1 (radians)2. xcex94n denotes the birefringence and xcex denotes the wavelength of the light.